Objective: Unsuspected idiopathic intracranial hypertension (IIH) is found in a significant minority of patients attending clinics with named headache syndromes, if it is specifically sought out. Chronic fatigue syndrome is frequently associated with headache. Could the same be true of chronic fatigue? Moreover, there are striking similarities between the two conditions. Could they be related? Attempting to answer these questions, we describe the results of a change in clinical practice aimed at capturing patients with chronic fatigue who might have IIH. Design: Cross-sectional. Setting: Hospital outpatient and radiology departments. Participants: Patients attending a specialist clinic with chronic fatigue syndrome and headache who had lumbar puncture to exclude raised intracranial pressure. Main outcome measures: Intracranial pressure measured at lumbar puncture and the effect on headache of cerebrospinal fluid drainage. Results: Mean cerebrospinal fluid pressure was 19 cm H2O (range 12–41 cm H2O). Four patients fulfilled the criteria for IIH. Thirteen others did not have pressures high enough to diagnose IIH but still reported an improvement in headache after drainage of cerebrospinal fluid. Some patients also volunteered an improvement in other symptoms, including fatigue. No patient had any clinical signs of raised intracranial pressure. Conclusions: An unknown, but possibly substantial, minority of patients with chronic fatigue syndrome may actually have IIH. An unknown, but much larger, proportion of patients with chronic fatigue syndrome do not have IIH by current criteria but respond to lumbar puncture in the same way as patients who do. This suggests that the two conditions may be related.